javascript - Javascript 数组未定义

Question

当我打印整个数组时，它是打印的。但是如果我尝试逐个元素打印，它会打印为未定义。这是我的函数。我在函数末尾打印数组。客户端函数用于连接ajax API。我尝试通过ajax函数从数据库获取与特定字符串匹配的整数id，并将它们推送到两个数组中。

function fetch() {
var arrayForClass = [];//this is a array get undefined at the end
var arrayForMessage = [];//this is a array get undefined at the end
exceptionPattern ="";
receivedData.length = 0;
var queryInfo;
var queryForSearchCount = {
    tableName: "LOGANALYZER",
    searchParams: {
        query: "_eventTimeStamp: [" + from + " TO " + to + "]",
    }
};

client.searchCount(queryForSearchCount, function (d) {
    if (d["status"] === "success" && d["message"] > 0) {
        var totalRecordCount = d["message"];
        queryInfo = {
            tableName: "LOGANALYZER",
            searchParams: {
                query: "_eventTimeStamp: [" + from + " TO " + to + "]",
                start: 0, //starting index of the matching record set
                count: totalRecordCount //page size for pagination
            }
        };
        client.search(queryInfo, function (d) {
             var obj = JSON.parse(d["message"]);
             if (d["status"] === "success") {
                 for (var i = 0; i < obj.length; i++) {
                     if(obj[i].values._level === "ERROR" || obj[i].values._level === "WARN"){
                         receivedData.push([{
                             date: new Date(parseInt(obj[i].values._eventTimeStamp)).toUTCString(),
                             level: obj[i].values._level,
                             class: obj[i].values._class,
                             content: obj[i].values._content,
                             trace: (obj[i].values._trace ? obj[i].values._trace : ""),
                             timestamp: parseInt(obj[i].values._eventTimeStamp)
                         }]);
                     }else{
                         continue;
                     }
                 }
                 console.log(receivedData);
                 for (forLoopI = 0; forLoopI < receivedData.length; forLoopI++){
                     var className = receivedData[forLoopI][0].class;
                     var strclassname = className.toString();
                     var messageContent = receivedData[forLoopI][0].content;
                     queryInfo = {
                         tableName: "EXCEPTION_CLASS_FOR_ERROR_PATTERNS",
                         searchParams: {
                             query: "class_name: "+ strclassname + "",
                             start: 0, //starting index of the matching record set
                             count: 1 //page size for pagination
                         }
                     };
                     client.search(queryInfo,function(d){
                         var obj = JSON.parse(d["message"]);
                         if (d["status"] === "success") {
                             var num = obj[0].values.id;
                             var strnum = num.toString();
                             arrayForClass.push(strnum);
                         }else{
                                 $(canvasDiv).html(gadgetUtil.getCustemText("No content to display","error while creating the error pattern" +
                                 " please try again"));
                         }
                     },function(error){
                          console.log(error);
                          error.message = "Internal server error while data indexing.";
                          onError(error);
                     });
                     queryInfo = {
                          tableName: "ERROR_MESSAGE_CONTENTS",
                          searchParams: {
                              query: "message: \""+ messageContent + "\"",
                              start: 0, //starting index of the matching record set
                              count: 1 //page size for pagination
                          }
                     };
                     client.search(queryInfo,function(d){
                          var obj = JSON.parse(d["message"]);
                          console.log(obj);
                          if (d["status"] === "success") {
                                  var num2 = obj[0].values.id;
                                  var strnum2 = num2.toString();
                                  arrayForMessage.push(strnum2);
                          }else{
                                 $(canvasDiv).html(gadgetUtil.getCustemText("No content to display","error while creating the error pattern" +
                                 " please try again"));
                          }
                     },function(error){
                          console.log(error);
                          error.message = "Internal server error while data indexing.";
                          onError(error);
                     });
                 }
            }
         }, function (error) {
             console.log(error);
             error.message = "Internal server error while data indexing.";
             onError(error);
         });
    }else{
        $(canvasDiv).html(gadgetUtil.getCustemText("No content to display","there are no error patterns which include this error" +
        " please try another one"));
    }
}, function (error) {
    console.log(error);
    error.message = "Internal server error while data indexing.";
    onError(error);
});
 console.log("------------------");
 for (var j = 0; j < 8; j++) {
    console.log(arrayForClass[j]);//prints undefine
 }
 console.log("------------------");
 console.log(arrayForClass[0]); //prints undefine
 console.log(arrayForClass);//prints corectly
 console.log(arrayForMessage);//printd corectly

}

Answer 1

您的 API 调用是异步的，这意味着即使您的调用尚未完成，它也会继续工作到下一行。

您得到未定义，因为您的console.log引用了尚未存在的变量。此时 arrayForClass 为空，因此 arrayForClass[0] 不存在。

下一行你会得到正确的结果，因为你 console.log 到一个现有的变量，即使它目前是空的，但你的调试器工具试图通过在控制台中为你更新它来变得聪明当您的数据传入时。

如果您确实想查看此时的实际值，则需要以某种方式使其不可变，例如:

console.log(JSON.parse(JSON.stringify(arrayForClass)));

这只是解释为什么你会像这样在控制台中获取数据。如果你需要使用这些变量，则必须在每次调用的回调函数中完成。